The present invention relates generally to ballasts compatible with chopper dimmers (e.g., TRIAC based dimmers). More particularly, the invention relates to methods and circuits for accurately detecting a conduction phase angle or OFF time introduced by a chopper dimmer.
Referring to FIG. 1, many residential and commercial lighting systems use chopper dimmers to modify power from an alternating current (AC) power source (e.g., 115V 60 Hz power line) to achieve dimmed operation of a light source. The chopper dimmer 110 operates with a phase conduction angle or OFF time to periodically block power from the AC power source 102 from reaching the ballast 104 as a function of a dimming level set by a user of the chopper dimmer 110. The chopper dimmer 110 may be a TRIAC dimmer, a DIAC dimmer, an insulated gate bipolar transistor (IGBT), or any other type chopper dimmer.
Referring to FIG. 2, the ballast 104 thus receives the line voltage 202 (i.e., the 115 VAC 60 Hz voltage from the AC power source 102) modified by an OFF time 204 or phase conduction angle equal to the dimming level. In the example shown in FIG. 2, the dimming level is 25% such that the chopper dimmer 110 blocks the line voltage 202 from the ballast 104 for 25% of the half period following each zero crossing of the line voltage 202. The chopper dimmer 110 varies the dimming level as a function of input from the user. The ballast 104 provides power to the light source 112 (i.e., lamps 106 and 108) as a function of the received voltage from the chopper dimmer 110. During the ON time 206 of the chopper dimmer 110, the modified line voltage 120 matches the line voltage 202.
Chopper dimmers are compatible with incandescent light sources, but for light sources that require ballasts (i.e., driver circuits) such as fluorescent lamps, high intensity discharge lamps, and solid state light-emitting devices (e.g., LED's and OLED's), the ballast must detect the OFF time or conduction angle and dim the light as a function of the detected OFF time. If the ballast 104 does not accurately detect the OFF time or conduction angle, then the ballast 104 will not accurately dim the light source 112.
Referring to FIG. 3, a prior art conduction angle sensing circuit 306 of the ballast 104 is shown. An electromagnetic filter 302 receives the modified line voltage 120 from the chopper dimmer 110, and provides it to a rectifier 304 of the ballast 104. The rectifier 304 provides a direct current (DC) voltage to a DC to DC converter of the ballast. The conduction angle sensing circuit 306 monitors the DC voltage output by the rectifier 304 to determine the conduction angle. The conduction angle sensing circuit 306 is a high impendence circuit. Due to its relatively high capacitance and resistance, the conduction angle sensing circuit 306 is not fast enough to sense relatively small OFF times associated with low dimming levels (e.g., 10%), particularly at low load values of the rectifier 304.